The invention relates to a fastening method for securely fastening two sheets of deformable material together. The method includes the formation of a rivet-like projection which is an integral part of the first sheet of material and which extends through an aperture in a second sheet of material.
Integral rivets have been used to fasten pull tabs to aluminum containers in the packaging industry. The materials used for these containers, including thin sheets of aluminum alloys, are more easily deformable than many other types of sheet material, including most other types of sheet metal. In making containers the object is to provide a fastener for a pull tab which is stronger than a container wall having a scoreline which defines a tear away portion so that as the tab is pulled a portion of the container wall will be removed. In other fields of manufacturing a much stronger fastener is needed for materials such as steel alloys which are not so easily deformable. These fasteners must withstand stress greater than that applied in the opening of an aluminum container.
In U.S. Pat. Nos. 3,404,648 and 3,198,155 two sheets of aluminum are deformed simultaneously to form a double layer integral rivet in a container wall. This method is not applicable to heavier or thicker materials which are less deformable than an aluminum container wall because it is not practical to deform a double thickness of such materials. In U.S. Pat. Nos. 3,688,385 and 3,637,106, a projection formed in a container wall is drawn through an aperture formed in a pull tab and is then bent down over the edges formed around the aperture. This method is not practical for heavier or thicker materials because a sufficient amount of material cannot be drawn to extend through the aperture to form a head which secures the two sheets of materials together. It is the practice in industries using heavier or thicker materials to make a projection in one sheet which is inserted through an aperture in a second undeformed sheet, and to then deform the projection in the aperture to produce a tight fit and to fasten the two sheets together. A problem with this method has been that the force required to deform the projection also deforms or enlarges the aperture which prevents a tight fit with the projection.